Sustainable biomimetic solar distillation with edge crystallization for passive salt collection and zero brine discharge

Water scarcity is a critical global issue exacerbated by population growth and unsustainable desalination practices. Conventional desalination methods, such as reverse osmosis and thermal desalination, are energy-intensive, reliant on fossil fuels, and produce significant brine waste, harming marine ecosystems. Direct solar vapor generation offers a sustainable alternative, harnessing solar energy for clean water production. However, salt accumulation on evaporator surfaces reduces efficiency. Existing solutions often involve salt diffusion back into the water reservoir, increasing salinity. This research proposes a biomimetic approach, mimicking mangroves' salt secretion and edge crystallization, to simultaneously produce freshwater and passively collect salt with zero brine discharge. The device is designed for scalability and uses low-cost, readily available materials.

Existing literature highlights the limitations of conventional desalination technologies in terms of energy consumption and brine rejection. Several studies have explored direct solar vapor generation as a more sustainable alternative, with various designs and materials proposed to enhance efficiency. However, many approaches struggle with salt accumulation, which hinders performance. Biomimetic designs inspired by natural systems like mangroves, which efficiently manage salt, have emerged as a promising avenue. Previous research has investigated bio-inspired structures for water purification and salt management, but few have integrated both functions effectively while achieving zero brine discharge.

The researchers designed a foldable, all-in-one mangrove-mimicked solar evaporator using chemically etched titanium mesh (TiO₂/Ti). The nanostructured TiO₂ layer provides excellent solar absorption, superhydrophilicity, and anti-corrosion properties. The device consists of a porous wicking stem and multi-layer leaves, mimicking the structure of a mangrove plant. The stem facilitates capillary-driven water transport, while the leaves serve as the evaporation surface. The experimental setup involved evaluating the device under simulated and outdoor sunlight conditions. The effects of various parameters, such as leaf tilt angle, stem height, solar irradiance, and brine concentration, were systematically investigated. The characterization methods included scanning electron microscopy (SEM), UV-vis-NIR spectrophotometry, infrared (IR) imaging, and contact angle measurements. A finite element method (FEM) simulation using COMSOL V5.6 was employed to model salt concentration distribution and back diffusion. The device's performance was evaluated in terms of evaporation flux, thermal efficiency, salt production, and water purity.

The mangrove-mimicked solar vapor generator (SVGC) demonstrated high performance. A tilt angle of +30° for the leaves optimized salt resistance and thermal efficiency. Under one sun, the device achieved a stable photothermal efficiency of approximately 94% when treating synthetic seawater (3.5 wt.% salinity). Outdoor experiments using real seawater resulted in a daily freshwater production of 2.2 L m⁻². The salt crystallized preferentially at leaf edges and peeled off passively during nighttime rewetting. Experiments with varying salinities (0-17 wt.%) showed that while evaporation rate decreased with increasing salinity, the device maintained relatively high efficiency. A double-layered device demonstrated improved performance compared to a single-layered one. The water produced met World Health Organization standards for drinking water. Notably, patchy salt accumulation on the leaves surprisingly enhanced evaporation rate in dark conditions, suggesting a contribution to overall performance.

The results demonstrate the successful integration of direct solar vapor generation and passive salt collection in a single, sustainable device. The biomimetic design, mimicking mangroves' efficient salt management, addresses a major limitation of conventional solar evaporators. The high efficiency and freshwater production rate, along with the zero brine discharge, position this technology as a viable solution for addressing water scarcity in regions with abundant sunlight. The passive salt collection mechanism simplifies the process and reduces operational costs. The enhanced evaporation rate observed with patchy salt highlights the unexpected benefits of the edge crystallization approach.

This study presents a sustainable and efficient biomimetic solar desalination device with zero brine discharge. The device achieves high photothermal efficiency and freshwater production, utilizing readily available materials. The passive salt collection mechanism simplifies operation and minimizes waste. Future research could focus on optimizing the device's design for different climates and scales, as well as exploring alternative materials for further cost reduction and improved durability.

While the device demonstrated high performance under controlled conditions, further research is needed to evaluate its long-term performance and robustness under varying environmental conditions. The scalability demonstrated in the four-device outdoor test needs to be validated on a larger scale. The study primarily focuses on seawater desalination; further investigation is required to assess its applicability to other saline water sources. The current model for salt transport assumes uniform porosity; improvements could be made incorporating more detailed material characterization.